1. Field of the Invention
The present invention relates to a video/audio decoding and reproducing apparatus which separates video data and audio data from a decoded signal including encoded video data and audio data, separately decodes the separated video data and audio data, respectively, and outputs the decoded video data and the decoded audio data. More particularly, the invention relates to a video/audio decoding and reproducing apparatus which is capable of effectively utilizing a storage means used at the time of decoding the video data. Also, the invention relates to a method of decoding and reproducing encoded video and audio data in the video/audio decoding and reproducing apparatus.
2. Description of the Related Art
In a video CD system, video data and audio data for a motion and/or a still picture having a pixel size of 352.times.240 (NTSC system) and audio, which are encoded in conformity with ISO/IEC 11172, are recorded on a recording medium (e.g., a video CD)
The video data and audio data conform to the video CD standards and ISO/IEC 11172, and are recorded as a data stream in which video packets, each having a packet header and encoded video data, and audio packets, each having a packet header and encoded audio data, are read out in a time series.
The data stream is read out from the thus-recorded recording medium by a reader (e.g., a configuration including a pickup and a CD-DSP (compact disk-digital signal processor)).
The data stream which has been read out by the reader is inputted to the video/audio decoding and reproducing apparatus. The video/audio decoding and reproducing apparatus separates the video data and audio data, respectively, from the inputted data stream, separately decodes the separated video data and audio data, and outputs the decoded data as decoded video data and decoded audio data.
A more detailed description will be given of the decoding of video data and audio data in the video/audio decoding and reproducing apparatus.
First, a description will be given of a case where the stream data which has been read out by the reader is the data stream for on a motion picture.
When data stream is inputted, the inputted data stream is separated into video data and audio data. The separated video data and audio data are temporarily stored in a storage means. The video data stored temporarily in the storage means is decoded by a video decoder, and the decoded video data is temporarily stored in the storage means.
In MPEG 1 (Moving Picture Experts Group Phase 1), the decoding of the video data by the video decoder comprises an intraframe encoded video (hereafter referred to as the I picture (intra-picture)), and an intraframe forwardly predictive coded video (hereafter referred to as the P picture (predictive-picture)), and a bidirectionally predictive coded video (hereafter referred to as the B picture (bidirectionally predictive-picture)). Hence, the video data on the I picture is decoded on the basis of the video data on the I picture which is sequentially stored temporarily in the storage means. This decoded video data on the I picture is temporarily stored in the storage means.
Next, on the basis of the video data on the P picture which is consecutively stored temporarily in the storage means, the video data on the P picture is decoded by referring to the decoded video data on the I picture stored temporarily in the storage means. This decoded video data on the P picture is stored temporarily in the storage means.
Subsequently, on the basis of the video data on the B picture which is consecutively stored temporarily in the storage means, the video data on the B picture is decoded by referring to the decoded video data on the I picture and the decoded video data on the P picture, which are stored temporarily in the storage means. This decoded video data on the B picture is stored temporarily in the storage means.
The decoded video data on the I picture, P picture, and B picture stored temporarily in the storage means is read by the video decoder at predetermined times, respectively. The video decoder outputs from a decoded video data output terminal video display data based on the decoded video data which has been read.
Meanwhile, the audio data stored temporarily in the storage means is decoded by an audio decoder so as to be outputted as decoded audio data which is decoded at the same time as the decoded video data in which the video data corresponding to the audio data is decoded. The decoded audio data is outputted from a decoded audio data output terminal. At this time, the storage means functions as a delay means for synchronization of the audio data with the decoding of the video data.
In addition, in a case where the stream data which is read out by the reader is data stream for on a still picture, the operation is as follows.
When data stream is inputted, the inputted data stream is separated into video data and audio data in the same way as the case of the motion picture. The separated video data and audio data are temporarily stored in the storage means. The video data stored temporarily in the storage means is decoded by the video decoder, and the decoded video data is temporarily stored in the storage means.
At this time, since the video data is one on the still picture, the decoded video data is generated without needing to refer to the decoded video data decoded before as in the case concerning the motion picture.
Therefore, the decoded video data stored temporarily in the storage means is read out by the video decoder at a predetermined timing. The video decoder outputs from a decoded video data output terminal video display data based on the vide o decoded data which has been read.
Meanwhile, the audio data stored temporarily in the storage means is decoded by the audio decoder so as to be outputted as decoded audio data which is decoded at the same time as the decoded video data in which the video data corresponding to the audio data is decoded. The decoded audio data is outputted from the decoded audio data output terminal.
In recent years, there has been a demand to record on a recording medium encoded video data and audio data for a high resolution still picture with the number of pixels per picture four times that of a motion picture or a still picture, i.e., 704.times.480 (NTSC system), and to decode the same by a video/audio decoding and reproducing apparatus.
However, the following problems are encountered in the video/audio decoding and reproducing apparatus configured as described above.
That is, since the decoded video data decoded by the video decoder is temporarily stored in the storage means, in the case of the high resolution still picture with the number of pixels per picture is four times that of a motion picture, the required storage capacity of the storage means becomes 4/3 times that of a motion picture. Accordingly, in order to make possible to decode the high resolution still picture as well, it is conceivable to enhance the storage capacity of the storage means by some means or other.
A more detailed description will be given of this aspect.
In the case concerning the motion picture, the storage capacity which is necessary for the storage means requires at least areas for respectively storing the encoded video data and audio data which have been read out from the storage medium (hereafter referred to as the video bit-stream area and the audio bit-stream area, respectively) and an area for storing the decoded video data decoded by the video decoder (hereafter referred to as the decoded video data motion picture area).
Then, the storage capacity imparted to one pixel in the decoded video data, i.e., the number of bytes (in this example, 1 byte=8 bits) is 1.5 bytes. That is, in the video format in the video CD standards, the color difference signals Cb and Cr are respectively 1/4 with respect to 1 of the luminance signal in the video format of MPEG 1, and 1 byte is allotted to the luminance signal per pixel, so that 1.5 bytes are allotted to one pixel.
Therefore, the volume of bytes per picture in the decoded video data motion picture area is 126,720 (=352.times.240.times.1.5) bytes, so that 380,160 (=126,720.times.3) bytes are required as the storage capacity of the decoded video data motion picture area.
In addition, about 37.5 kilobytes are required as the storage capacity of the video bit-stream area in the MPEG 1 standards. The storage capacity of the audio bit-stream area may be smaller than the storage capacity of the video bit-stream area, but a predetermined volume of storage capacity is required.
Accordingly, as the storage means for the motion picture, it is sufficient to use a memory of 512 kilobytes (words).times.8 bits, specifically a 4M DRAM.
In addition, in the case concerning the still picture, the storage capacity which is necessary for the storage means requires at least the video bit-stream area, the audio bit-stream area, and an area for storing the decoded video data decoded by the video decoder (hereafter referred to as the decoded video data still picture area).
However, since the storage capacity of the decoded video data still picture area may be a capacity which is capable of storing the decoded video data of a one-picture portion, the storage capacity of the storage means can be smaller than the case concerning the motion picture, and the storage means for the motion picture, specifically the 4M DRAM, may be used as it is.
On the other hand, in the case concerning the high resolution still picture, the storage capacity of the area for storing the decoded video data decoded by the video decoder (hereafter referred to as the decoded video data high resolution still picture area) requires 506,880 (=704.times.480.times.1.5) bytes.
Accordingly, if the storage means for the motion picture, specifically the 4M DRAM, is used, the storage capacity of the storage means other than the decoded video data high resolution still picture area is 5,120 bytes.
Therefore, if an attempt is conceivably made to add the storage capacities of the video bit-stream area and the audio bit-stream area to the storage means, it is necessary to newly add one memory as the storage means, e.g., a 1M DRAM or a 4M DRAM, or to change the memory to one having a large storage capacity, e.g., a 16M DRAM. Hence, the addition of the memory or the change to a memory having a large storage capacity entails an increase in the mounting area in the video CD system including the video/audio decoding and reproducing apparatus, leading to a higher cost.